Semiconductor image sensors have been widely utilized, as the fine structures required by the images sensor are easily processed by conventional semiconductor processing techniques and the semiconductor image sensors are easily incorporated in various devices by virtue of high density integration with their pertinent circuits in a semiconductor chip. Since it is necessary to incorporate the image sensor or sensors with optical means such as lenses etc. for image detection, the image sensor or sensors are very often incorporated with the optical means into a module. In application to auto-focussing cameras, it is necessary to incorporate, as is well known, a pair of image sensors in a camera for detecting a distance between the camera and a subject or for detecting focussing level. FIG. 3 is a sectional view showing a main portion of a module into which the optical means and a pair of image sensors are incorporated.
A semiconductor apparatus 70 shown in the lower portion of FIG. 3 is comprised of a chip 71 into which a pair of right and left image sensors 10R and 10L and their pertinent circuits are incorporated; a die plate 72 of a lead frame on which the chip 71 is mounted as usual; and a package 75, for example of transparent resin, in which the die plate 72, the chip 71 and bonding wires 74 connecting the chip 71 and leads 73 are sealed. The image sensor 10R or 10L is further comprised of a plurality, e.g. 128 pieces, of optical sensing elements. So-called charge storage type optical sensing elements are used for the sensor 10R or 10L for detecting an image of a subject as precisely as possible, for example in the auto-focussing camera.
An optical means 80 is comprised of a small transparent plastic mold in which a pair of lenses 80R and 80L which focus the images of the subject on the image sensors 10R and 10L. The optical means 80 is bonded, e.g. by adhesion, with the semiconductor apparatus 70 into a module through an opaque plastic mold adaptor 81. The images of the subject through the lenses 80R, 80L and internal spaces 81a of the adaptor 81 are formed on the image sensors 10R, 10L separated from the lenses 80R, 80L by their focussing length.
Though the image detection precision has been improved by the use of the charge storage type optical sensing elements for the image sensors, a much higher precision has been required recently in image detection such that it is now very hard to attain the required precision by the module in which the image sensors and the optical means are incorporated. Though the optical sensing element for auto-focussing camera is now required to convert the received light intensity with the precision of 8 bits, i.e. with an error of less than 1%, to the image data, the conversion precision according to the prior art remains at 5 or 6 bits, i.e. an error of from 2 to 3%.
Though this limitation is caused by the deviation of optical sensitivity between many optical sensing elements, the limitation is mainly caused by the deviations in relative position and attitude of the image sensors and the optical means from the ideal or designed position and attitude which is further caused by the process through which the image sensors and the optical means are incorporated. FIG. 4 is a diagram for explaining the distributions of the image data DR and DL obtained by the optical sensing elements in the image sensors 1OR and 10L when the optical means 80 is irradiated with a uniformly distributing light. In FIG. 4, the distribution of the image data DL with the i-th optical sensing element in the image sensor 10L is shown on the left hand side of the figure with i varying from l to m, and the distribution of the image data DR with m pieces of the optical sensing elements in the image sensor 10R is shown on the right hand side of the figure. The distributions of the image data are expanded in FIG. 4 for clarity except small variations in the image data DR or DL of the individual optical sensing element.
As shown in FIG. 4, though the image data DR and DL show upward concave quadratic distributions, the difference Dc between the average levels of the image data DR, DL from the image sensors 10R, 10L is more conspicuous than the quadratic distributions of the image data. The difference Dc between the average levels of the image data DR, DL is obviously caused not by the detection sensitivity variation between the optical sensing elements, but by the difference between the quantity of light received by the image sensors 10R and 10L through the optical means 80.
In view of the foregoing, an object of the present invention is to provide an image data circuit for a pair of image sensors which compensates the above described difference between the distribution levels of the image data obtained with the image sensors which receive images of a subject from the optical means.